The Collaborative International Dictionary of English v.0.48:

Hunker down \Hun"ker down\, v.
   1. to crouch or squat; to sit on one's haunches.
      [PJC]

   2. to settle in at a location for an extended period; -- also
      (figuratively) to maintain a position and resist yielding
      to some pressure, as of public opinion.
      [PJC]

   3. to take shelter, literally or figuratively; to assume a
      defensive position to resist difficulties. "We hunkered
      down to ride out the storm in an abandoned cabin."
      [PJC]

            While many businessmen were hunkering down for
            another bust after the lean years of the Second
            World War and the Great Depression before it, Taylor
            and company correctly reckoned it was the dawn of an
            era of prosperity and growth.         --Richard
                                                  Siklos [Shades
                                                  of Black,
                                                  1995]
      [PJC]

WordNet (r) 3.0 (2006):

hunker down
    v 1: sit on one's heels; "In some cultures, the women give birth
         while squatting"; "The children hunkered down to protect
         themselves from the sandstorm" [syn: squat, crouch,
         scrunch, scrunch up, hunker, hunker down]
    2: take shelter; "During the sandstorm, they hunkered down in a
       small hut"
    3: hold stubbornly to a position; "The wife hunkered down and
       the husband's resistance began to break down"